Article pubs.acs.org/IECR
Hydroconversion of 2‑Methylnaphthalene on Pt/Mordenite Catalysts. Reaction Study and Mathematical Modeling Horacio González,*,† Omar S. Castillo,† José L. Rico,† Aída Gutiérrez-Alejandre,‡ and Jorge Ramírez‡ †
Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Edificio M, Ciudad Universitaria, 58060, Morelia, Michoacán, México ‡ UNICAT, Facultad de Química, UNAM, Ciudad Universitaria, México D.F. S Supporting Information *
ABSTRACT: In this study, the effect of the metal/acid balance of Pt/mordenite catalysts on the activity, product distribution, and main reaction pathways during the hydroconversion of 2-methylnaphthalene (2-MN) was analyzed. The hydroconversion study was performed on Pt/mordenite catalysts having different metal/acid ratios in a batch reactor at 4.13 MPa (cold charge) and 523−573 K. The catalysts were characterized by XRD, 27Al NMR spectroscopy, FTIR spectroscopy of adsorbed pyridine, and TEM. The results indicate that, by using a bifunctional catalyst consisting of a well-crystallized mordenite whose acid function consists mainly of Brönsted acidity and with a strong hydrogenating function such as dispersed platinum, it is possible to promote the hydroconversion of 2-MN through a complex reaction scheme consisting of isomerization, hydrogenation, ring contraction, ring opening, and cracking. These reactions produce high-value hydrocarbons such as alkyl cycloparaffins, paraffins, and isoparaffins, which decrease the aromatic character of the feed and open a potential route to improve the quality of the polyaromatic-rich fractions.
1. INTRODUCTION Catalytic hydroconversion is an attractive secondary conversion process in the modern refinery for achieving more valuable products, as well as cleaning the atmosphere from heavier petroleum fractions. For example, the light cycle oil (LCO) fraction, a byproduct from catalytic cracking, is normally high in sulfur, nitrogen, and particularly polyaromatics. This petroleum cut, usually in a boiling range similar to that of diesel, can be used as a diesel-blending component only after hydrotreating.1,2 LCO is rich in multiring aromatics, which will produce fuel with low cetane number. Upgrading LCO to higher-cetane-number fuels or to hydrocarbons in the gasoline boiling range requires hydrogenation of multiring aromatics followed by ring-opening reactions.3,4 Therefore, a bifunctional metal/acid catalyst is necessary to promote those hydroconversion reactions. Metal/ acid zeolite catalysts are bifunctional catalysts that are widely used in petroleum refining, especially in hydrocracking and hydroisomerization processes.1,5 Highly active sulfur-resistant catalysts have been developed, mainly based on Pt, Pd, or mixtures of the two, supported on acidic carriers.6,7 On these catalysts, the transformation of hydrocarbons involves hydrogenation and dehydrogenation steps on metal sites and rearrangement and/or cracking steps on acid sites, plus the step of moving the reaction intermediates from metal to acid sites.8 Therefore, the number and characteristics of the metal and acid sites will determine the activity and selectivity of bifunctional catalysts. Indeed, it has been shown that the balance between metal and acid sites strongly influences the performance of bifunctional catalysts in the hydroconversion of different hydrocarbons such as n-paraffins, naphthenes, and aromatics.9−12 However, studies regarding the hydroconversion of polyaromatics and the effect of the metal/acid balance on the activity and selectivity are scarce.13−15 In previous studies, © 2013 American Chemical Society
metal/zeolite bifunctional catalysts were used for the hydroconversion of different hydrocarbons. For example, Moreau et al.16,17 studied ortho-xylene and ethylbenzene hydroconversion on mechanical mixtures of Pt/alumina + mordenite with different metal/acid ratios. In some other studies, the metal/ acid ratio of bifunctional metal/zeolite catalysts was modified to achieve a good balance between the metal and acid functions during the isomerization and hydrocracking of paraffins.9,11,18−20 However, currently, in the open literature, we could not find similar works related to the hydroconversion of 2-MN on Pt/mordenite with different metal/acid ratios. In this study, we analyzed the effects of the metal/acid balance of Pt/mordenite catalysts on the activity, product distribution, and main reaction pathways during the hydroconversion of 2-MN. A simplified reaction model is also proposed to explain the product distribution of the main groups of hydrocarbons.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Hydroconversion experiments were performed over prereduced Pt/mordenite (Pt/HMOR) catalysts. A commercial ammonium mordenite in powder form, with a SiO2/Al2O3 molar ratio of 20, was acquired from Zeolyst. Calcination of this sample for 2 h at 673 K led to the acid form of the zeolite (HMOR). The manufacturer of this zeolite reported a Na2O content of 0.08 wt % and a surface area of 500 m2/g. The metal/acid balance of the catalyst was modified by changing the metal content (0.05−1.0 wt % Pt) while keeping Received: Revised: Accepted: Published: 2510
August 13, 2012 December 16, 2012 January 14, 2013 January 14, 2013 dx.doi.org/10.1021/ie302170v | Ind. Eng. Chem. Res. 2013, 52, 2510−2519
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the acid function fixed. The obtained catalysts are denoted Pt(x %)/HMOR, where x is the weight percent of platinum in the catalyst. Platinum was added to the zeolite by incipient wetness impregnation, using solutions of [Pt(NH3)4](NO3)2 at the appropriate concentrations for obtaining the required metal loadings. The impregnated samples were dried at 393 K overnight in air and then calcined at 673 K for 4 h at a heating rate of 2.77 K min−1. 2.2. Reaction Study. Catalytic experiments were conducted in an electrically heated 500-mL stainless-steel batch reactor (Parr instruments 4570/80) operating at 600 rpm in the presence of hydrogen at 523−573 K and 4.13 MPa (cold charge). First, the calcined catalyst was reduced ex situ in a quartz-glass-packed continuous-flow reactor at 673 K at a heating rate of 5 K min−1 for 3 h, using a hydrogen flow of 10 mL/min. After reduction, H2 gas was replaced by N2 as the temperature decreased to room temperature. Next, the reactor was charged with 250 mL of a solution containing 3 wt % of 2MN in n-decane. Both catalyst and reactor system were maintained in a sealed chamber under an argon atmosphere to avoid Pt oxidation. Then, the catalyst was added quickly to the reaction mixture, and the reactor was sealed and then purged with H2. The amount of catalyst used in each experiment, expressed as a ratio (model compound/catalyst), was 15 g/g. Afterward, the reactor was pressurized with hydrogen to 4.13 MPa and heated to the experimental temperature in 10−15 min; the final pressure varied from 6.89 to 7.58 MPa. Once the reaction temperature had been reached, a sample was collected from the reactor, and this was taken as the zero time of the reaction test. The analysis of the reaction products was performed by gas chromatography (GC) in a Hewlett-Packard HP-5890 chromatograph provided with a flame ionization detector (FID). The heating rate in the chromatograph was 1 K/min to 423 K and then 5 K/min to 473 K. The reaction products were identified by mass spectrometry (MS) on a Leco Pegasus 4D instrument. Sample analysis was performed with a DB-5 capillary column (length, 50 m; diameter, 0.2 mm; film thickness, 0.33 μm). The same column was used for identification by GC-MS analysis. The mass spectrum obtained for each compound was compared with the 2005 version of the NIST mass spectrum library. 2.3. Catalyst Characterization. The crystal structures of some representative catalysts were analyzed using powder X-ray diffraction (XRD). The diffractograms were obtained with a Siemens D5000 diffractometer in the 2° ≤ 2θ ≤ 70° range using graphite-monochromated Cu Kα radiation and a gyrometer speed of 1.0°/min. 27Al nuclear magnetic resonance (27Al NMR) spectroscopy was used to analyze the coordination of the aluminum present in the zeolite and the possible changes occurring during catalyst preparation. 27Al NMR studies were performed in a Bruker ASX300 spectrometer for solid samples. The acidic properties of HMOR and Pt(x%)/HMOR were studied by FTIR spectroscopy of adsorbed pyridine. FTIR spectra were recorded on a Nicolet Magna 760 spectrophotometer using very thin pellets of the samples, which were activated by outgassing the cell at 723 K for 2 h. The IR experiments were performed using a homemade glass cell with CaF2 windows, connected to a conventional gas-manipulation/ evacuation apparatus. The adsorption experiments consisted of 3 min of contact of the activated sample with pyridine (10 Torr), which allowed saturation of the available surface. Then, evacuation was conducted at different temperatures (373, 473, and 573 K), and spectra were obtained after each evacuation.
The catalysts were also analyzed by transmission electron microscopy (TEM) to study the distribution and particle size of the impregnated platinum. Microscopy analysis was performed on a JEOL 2010 FEG field-emission transmission electron microscope with 0.19-nm point resolution. Images of platinum particles in the samples were obtained with atomic resolution by the Z-contrast method. The carbon contents of spent Pt(x %)/HMOR catalysts with 0.05, 0.5, and 1.0 wt % Pt after 9 h of reaction at 548 K were determined by elemental analysis using a Perkin-Elmer PE2400 analyzer. Before analysis, the spent catalysts were washed with n-decane using an ultrasonicator for 10 min and then dried at 673 K for 1 h under a flow of nitrogen to remove the distillable products.
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. X-ray Diffraction. Figure 1 shows the diffraction pattern of the HMOR zeolite, which
Figure 1. XRD diffraction pattern of the HMOR zeolite. From the 430171 JCPDS card, eight main peaks were identified, which are located at 2θ = 9.76°, 13.52°, 19.70°, 22.42°, 25.73°, 26.44°, 27.64°, and 27.87°.
was compared with JCPDS card 43-0171, confirming that the locations and relative intensities of the main peaks corresponded well to those of mordenite. Furthermore, in Figure 1, one can observe that the zeolite has a high degree of crystallinity, as peaks in the diffractogram are well-defined and no significant amounts of amorphous material can be observed. A similar analysis was performed on the Pt(1%)/HMOR sample, and no appreciable changes were found in the XRD pattern (not shown), because of the low platinum loadings used in the study. Therefore, after the different treatments, the zeolite used in this study maintained its crystalline structure. 27 Al Nuclear Magnetic Resonance Spectroscopy. The 27Al NMR characterization technique was used to determine the coordination of aluminum in the HMOR and Pt(1%)/HMOR samples. The 27Al NMR spectrum of HMOR presented in Figure 2a shows a single appreciable signal at 55.7 ppm, which corresponds to tetrahedrally coordinated aluminum.21 However, no signal associated with aluminum in octahedral coordination (usually located close to 0.0 ppm) was observed. This indicates that aluminum in tetrahedral coordination is predominant in HMOR. As mentioned in the literature, each aluminum atom in tetrahedral coordination is associated with a Brönsted acid site.22 Because a greater amount of tetrahedral aluminum is present in the zeolite, it can be inferred that the acid properties of HMOR are mainly provided by Brönsted acid sites. Figure 2b shows the 27Al NMR spectrum of the Pt(1%)/ 2511
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frequently used to characterize the Lewis and Brönsted acidities of acid catalysts. The absorption band located at 1452 cm−1 is associated with coordinatively bonded pyridine adsorbed on Lewis acid sites, whereas the absorption band at 1543 cm−1 corresponds to protonated pyridine adsorbed on Brönsted acid sites.23,24 On the other hand, the band located at 1488 cm−1 corresponds to pyridine adsorbed on both Brönsted and Lewis acid sites. The bands at 1452 and 1543 cm−1 are useful for determining the predominant acidity of zeolites.25 As evidenced in the IR spectra, the absorption band at 1543 cm−1 is more intense than the band located at 1452 cm−1, indicating that Brönsted acid sites are predominant in HMOR. These results agree well with those obtained by 27Al NMR spectroscopy, where it was found that the aluminum in HMOR is mainly in tetrahedral coordination. Because each tetrahedral aluminum atom is associated with a Brönsted acid site, it can be inferred that the acidity of the zeolite is predominantly of the Brönsted type. For Pt(x%)/HMOR catalysts, a small decrease in the intensity of the IR bands associated with Brönsted and Lewis sites (not shown) was observed, indicating that the incorporation of platinum had a small effect on the numbers of Brönsted and Lewis acid sites of the HMOR zeolite. Because impregnation was the method used for Pt introduction, it is expected that the Pt particles were mainly located on the outer surface of the zeolite, as confirmed by the average size of platinum particles (1.5 nm) determined by TEM. However, at least a small part of Pt particles seem to have been located in the cavities of the zeolite. Transmission Electron Microscopy (TEM). Atomic-resolution images of platinum particles in the samples were obtained by Z-contrast to determine their size on the impregnated Pt(x %)/HMOR catalysts. Figure 4 shows images of platinum particles impregnated over different catalysts. DigitalMicrograph 3.7 software, which is suitable for analyzing atomicresolution micrographs, was used to measure the sizes of the platinum particles. Analysis of the platinum particles in 12 micrographs gave an average particle size of 1.5 nm (15 Å) in the samples. This result indicates good dispersion of the platinum particles on the zeolite and is in line with the results for similar systems, where metallic particles smaller than 3 nm are considered to be well dispersed on the supports.26 Given that the impregnated platinum particles had an average size of 1.5 nm and that the diameter of the 12-membered channels of mordenite is 0.74 nm, it can be inferred that the platinum particles were mainly located on the external surface of mordenite and only a small fraction of platinum might have been located within the zeolite channels. As discussed before, because the amounts of impregnated platinum in this study were relatively small (0.05, 0.5, and 1.0 wt %), the metallic particles did not have a significant effect on the acidity of the zeolite, in line with the results of FTIR analysis and related studies.27,28 Although the catalysts were subjected to different treatments, the platinum particles did not undergo any significant changes in size, as can be seen in micrographs b and c of Figure 4 for the Pt(0.5%)/HMOR catalyst before and after reduction, respectively. Similarly, the platinum particles did not undergo a significant modification in size and dispersion after reaction, as can be seen in micrograph d of Figure 4 for the Pt(1%)/ HMOR catalyst. Carbon Content in Spent Catalysts. Figure 5 shows the amounts of carbon deposited on spent Pt(x%)/HMOR catalysts as a function of platinum content. The hydro-
Figure 2. 27Al NMR spectra for different samples: (a) HMOR, (b) Pt(1%)/HMOR.
HMOR catalyst after calcination. No appreciable changes can be observed compared to the spectrum of the HMOR sample, indicating that the preparation method did not affect the coordination of aluminum in the zeolite. FTIR Spectroscopy of Adsorbed Pyridine. Figure 3 shows the FTIR spectra of pyridine adsorbed on HMOR in the 1400− 1600 cm−1 region at different temperatures of evacuation. In this figure, one can see the two absorption bands that are
Figure 3. FTIR spectra of adsorbed pyridine over HMOR after evacuation at (a) 373, (b) 473, and (c) 573 K. 2512
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Figure 5. Amounts of carbon deposited on spent Pt(x%)/HMOR catalysts as a function of Pt content. The hydroconversion experiments were performed at 548 K for 9 h.
work (lower than 3.5 wt %). This percentage decreased as the amount of platinum in the catalyst increased; indeed, sample Pt (1%)/HMOR presented the lowest carbon content (1.17 wt %). These results indicate that Pt promotes the hydrogenation of carbon precursors in the catalyst, which is in line with the findings of previous studies.29,30 The spent catalyst with the greatest deposition of carbon (3.26 wt %) was that with the lowest metal content Pt(0.05%)/HMOR, in agreement with its lower metal/acid ratio and the lower expected hydrogenation of carbon precursors. In fact, acidic mordenite has been reported to be highly susceptible to coking and deactivation in different hydrocarbon reactions.31−33 This behavior has been attributed to the pore structure of HMOR, because larger hydrocarbon molecules can diffuse in only one direction in its pore system. However, for this particular reaction, the Pt(x%)/HMOR samples studied here showed reasonably good stability. This is mainly because the coking rates in hydroconversion reactions are lower, as the coke precursors can be hydrogenated by the metallic component and then desorbed. Furthermore, the formation of bulky carbon precursors in the zeolite pores is quite difficult because there is not enough space for such molecules to form.29 3.2. Hydroconversion of 2-MN. To assess the reproducibility of the catalytic experiments, at the beginning of the study, a hydroconversion reaction was performed using 2-MN (3 wt %) in n-decane as the reactant over Pt(1%)/HMOR at 523 K and 41.3 MPa (cold charge). This experiment was repeated twice, with a variation in conversion of less than 2.2% and a similarly small variation in the percentages of products ( Pt(0.05%)/HMOR. Figure 6 shows the conversion of 2-MN versus reaction time for these catalysts.
Reactivity of the Solvent. To analyze the reactivity of the solvent (n-decane) during the hydroconversion of 2-MN, a reaction experiment was performed using n-decane as the feed at 523 K and 4.13 MPa (cold charge) over Pt(0.5)/HMOR. The conversion of n-decane was quite low (0.8 wt % at 8 h) at these operating conditions (Figure S1 in the Supporting Information), indicating that n-decane exhibits a low reactivity. The product distribution resulting from n-decane conversion included mostly low-molecular-weight compounds such as paraffins and isoparaffins. In the Supporting Information (Table S1), the yields of the main products from n-decane hydroconversion are presented, where yield is defined as the weight percentage of a particular product divided by the sum of the weight percentages of all products and multiplied by 100. The main products were found to be paraffins and isoparaffins, including methylnonanes, dimethyloctanes and ethyloctanes, propylheptanes, hexane, methylpentanes, dimethylbutane, and butane. Unidentified products were mainly heavy hydrocarbons containing more than 10 carbon atoms (C10+). Based on these results, it can be inferred that the main reactions occurring during n-decane conversion are isomerization and cracking. Typical Product Distribution in the Hydroconversion of 2MN. The yields of the main individual products obtained at 523 K and 9-h reaction time from 2-MN hydroconversion over Pt(1%)/HMOR and Pt(0.5%)/HMOR are presented in the Supporting Information (Table S2). Most of the products were clearly identified (>95%), although greater difficulty was experienced in identifying some heavy products. According to the product distribution obtained from 2-MN hydroconversion, the primary products resulting from 2-MN hydrogenation were found to be mainly methyltetralins, with 5methyltetralin being the hydrogenated product present in the highest percentage. Also present were significant amounts of five-membered saturated ring products with alkyl substituents, such as methyl-, ethyl-, and dimethylindans, which are produced through the ring contraction of methyltetralins. A significant amount of alkylbenzenes was also detected, which probably came from ring-opening reactions of bicyclic compounds with a saturated ring (methyltetralins and alkylindans). Other important products included 2-MN isomers, mainly 1-methylnaphthalene. This last result is in line with related studies regarding the conversion of substituted naphthalenes, where high yields of methylnaphthalene isomers were observed as a result of the fast isomerization of the alkyl group.36,37 Products containing more than one alkyl substituent such as dimethylnaphthalenes, trimethylnaphthalenes, methylethylnaphthalenes, and other heavier compounds were also present and might have been formed through disproportionation and alkylation reactions. Finally, significant amounts of paraffins, isoparaffins, cycloparaffins, and alkyldecalins were also present in the product distribution; they can be associated with several carbocation rearrangements such as isomerization, ring contraction, ring opening, and cracking. The results of the carbon balance are also presented in Table S2 (Supporting Information). The deviation from the carbon balance of the charge to the reactor was under 5% for each of the hydroconversion experiments. 3.3. Effect of the Catalyst Metal/Acid Balance on the Activity and Product Distribution. Because of the large amount of products generated in the hydroconversion of 2MN, it seems convenient to group them to facilitate their analysis. Therefore, the reaction products were divided into the five groups as follows:
Figure 6. 2-MN conversion versus reaction time for Pt(x%)/HMOR catalysts at 523 K.
The catalyst with the lowest metal/acid ratio [Pt(0.05%)/ HMOR] produced the lowest conversion of 2-MN among the catalysts studied here, as a consequence of the small amount of metal on the catalyst, limiting the hydrogenation and dehydrogenation reactions taking place over the metallic sites. It is well-known that the hydroconversion of a polyaromatic compound begins with the saturation of one of its rings on the metallic sites, followed by contraction, ring opening, dealkylation, and isomerization over the acidic sites.6,25 As can be seen in Figure 6, when the metallic content in the catalyst was increased, the catalytic activity is increased markedly, leading to a higher conversion of 2-MN. Figure 7 shows the product distribution by groups for Pt(x %)/HMOR catalysts with different metal/acid ratios. The catalyst with the lower metal/acid balance (Figure 7a) 2514
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MN is a key step in the generation of these product groups that include valuable hydrocarbons. On the other hand, the production of PC11+ compounds (mainly alkylnaphthalenes) with this catalyst was very low and reached a maximum around 5 h; subsequently, its production decreased to very low yields. This is a consequence of different secondary reactions that are favored at longer residence times, such as deep hydrogenation and different carbocation rearrangements. Additionally, in Figure 7b, it can be seen that the production of light hydrocarbons (LP group) increased only slightly with this catalyst. To properly compare the performance of the different catalysts, Figure 8 shows the yield of the different groups of
Figure 8. Yields of the different groups of products at similar conversions of 2-MN (25%). Hydroconversion of 2-MN over Pt(x %)/HMOR catalysts at 523 K for elapsed times varying from 4.4 to 7 h.
Figure 7. Product distribution by groups (wt %) versus reaction time at 523 K. 2-MN hydroconversion over (a) Pt(0.05%)/HMOR and (b) Pt(1%)/HMOR.
products at similar conversions of 2-MN (25%). In this figure, it is evident that the catalyst with the lowest metal/acid ratio Pt(0.05)/HMOR was the one with the highest selectivity toward PC11+ products. As mentioned previously, the higher proportion of acid sites in this catalyst favored the production of alkylnaphthalenes. When the metal/acid ratio of the catalyst was increased [Pt(1%)/HMOR], the selectivity toward groups AI and AB increased, indicating that the higher concentration of metallic sites on the catalyst favored the production of compounds with one or two saturated rings, which subsequently reacted on the acid sites, where different carbocation rearrangements occurred, producing high-value compounds. Moreover, this catalyst also produced the lowest amount of high-molecular-weight hydrocarbons (PC11+ group) and a moderate amount of light compounds (LP group). Figure 9 shows the product distributions (in terms of weight percentage) of groups AB, AI, and PC11+ versus platinum
produced a high amount of MT-group products at short reaction times, reaching a maximum at 8 h. As mentioned before, this group also contains 1-methylnaphthalene, which is generated in higher percentages and is formed through the isomerization of 2-MN on the acidic sites of the catalyst. Therefore, with the Pt(0.05%)/HMOR catalyst, the evolution of the MT group is partially associated with the behavior of 1methylnaphthalene. Figure 7a also shows a gradual increase in the percentage of the PC11+ group that is accentuated at longer reaction times (19% at 8 h to 29% at 9 h). This increase in the amount of PC11+ products could be associated with secondary reactions of the compounds present in the MT, AI, and AB groups. In the same figure, one can see that the maximum percentage achieved by the AB group was quite low (
